Multiple myeloma (MM) is the 2nd most common, age related, hematological malignancy with an estimated 30,000 new cases and 13,000 deaths per year. Patients with upfront refractory disease progress fast, and those that enter remission eventually die as a consequence of relapsed disease. To increase patient survival, clinicians need new ways of selecting effective and durable therapies for relapsed and refractory MM patients. Antibody (Ab) based treatments that target cluster of differentiation 38 (CD38), a transmembrane protein consistently expressed in MM cells, is a promising therapy for relapsed and refractory MM patients. Currently, there are three CD38 targeted monoclonal Abs (mAbs) in the clinic for MM patients, daratumumab, isatuximab and MOR-202. These Abs are safe, well tolerated and show remarkable efficacy in ~30% of myeloma patients. It is unknown why the majority of remaining patients do not respond to CD38 targeted mAb therapy and why there is such variability between patients. Functionally, CD38 is a pleiotropic antigen that acts as an enzyme as well as a receptor. The extracellular enzymatic component of CD38 is responsible for the metabolism of nicotinamide adenine dinucleotide (NAD) to downstream Ca2+ signaling molecules. There is growing evidence that variability in CD38 targeted therapy response in MM cells could in large part be contributed to the changes in its NAD enzymatic activity. Here, we posit that while CD38 expression is important, the monitoring of CD38 enzymatic activity in concert with spatiotemporal imaging is key to linking MM response to CD38 targeted therapies. We propose to address this question using Positron Emission Tomography (PET), which is a powerful molecular imaging tool that can be directly applied to quantify receptor expression and functional activity in vivo, non- destructively and longitudinally. We have shown that PET imaging of CD38 antigen on MM cells is feasible using the radiolabeled CD38 mAb daratumumab. A limitation of this approach however is that Abs are not suited for longitudinal imaging studies due to their long biological half-life. High-affinity peptide based small molecule imaging agents can effectively overcome these limitations since they are rapidly taken up by tumor tissue while being promptly cleared from non-target organs in vivo. Despite the unmet need, there is a paucity of CD38 targeted small molecule peptide based PET probes for MM. Using innovative phage display technology, we will develop new CD38 targeted peptide-based PET imaging probes that are specific for enzymatically activate CD38 conformation. Furthermore, in highly relevant biological systems, we will evaluate molecular pathways as well as spatial relationships that govern MM-associated CD38 expression. The specific aims are: (1) Aim 1a. Identify and validate changes in CD38 expression and enzymatic activity before and after CD38 targeted therapies in human mouse models of MM. Aim 1b. Evaluate spatiotemporal CD38 expression based on PET imaging. (2) Aim 2. Develop, evaluate and optimize novel high affinity CD38 targeted peptide PET probes. In summary, the current proposal is innovative and potentially clinically transformative since it seeks to identify those MM patients who are most sensitive and most resistant to anti-CD38 monoclonal immunotherapy.